1. Field of the Invention
The present invention relates to a frequency control device and method for frequency synchronization with a multiplex signal using OFDM (Orthogonal Frequency Division Multiplexing), a receiving device, and a communication device, and in particular to a frequency control device and method, receiving device, and communication device which are suitable for frequency synchronization with a multiplex signal having deviation not less than half a sub-carrier interval.
2. Description of Related Art
As a modulation system in digital communications, the OFDM system has found extensive practical use.
As an example of systems employing OFDM, there can be listed the EUREKA-147 SYSTEM, which is generally called DAB (Digital Audio Broadcasting), the EURELA-147 DAB System, etc. In the following, we will use the "EUREKA-147 DAB System". In November, 1994, ITU-R (International Telecommunication Union-Radio communication Sector) admitted EUREKA-147 DAB System as System-A, and it has become an international standard. This standard is issued as "ETS 300401".
In OFDM systems, data are multiplexed by dividing them onto a plurality of sub-carriers which are quadrate to one another. A baseband frequency of each sub-carrier is selected to be an integer multiple of a certain fundamental frequency. Assuming that one cycle of the fundamental frequency is a significant symbol duration, a product of sub-carriers that are different from each other is 0 when integrated in the significant symbol interval. In that case, it is said that these sub-carriers are quadrate.
In OFDM systems, when differences arise in frequency between sending and receiving sides, orthogonality to the other sub-carriers cannot be maintained at the time of demodulating. Owing to interference, this becomes the cause of errors in the demodulated data. As causes of the above-described frequency difference, there may be listed, for example, error and variation of an oscillation frequency of a reference oscillator on each of the sending and receiving sides, Doppler shift due to relative movement between the sending and receiving sides, and the like.
To obtain demodulated data with a smaller number of errors even when frequency difference is produced, frequency synchronization systems have been studied. For example, such a frequency synchronization system is described in "A New Frequency Synchronization Technique for OFDM Demodulators Using Guard Intervals", (ITEJ(Institute of Television Engineers of Japan) Technical Report, Vol. 19, No. 38, pp. 13-18). This frequency synchronization system utilizes the guard interval in which a signal waveform in the significant symbol duration is repeated cyclically.
Namely, a received signal is translated into the baseband through a quadrature detection circuit, and each carrier component is then demodulated by a FFT (Fast Fourier Transform) circuit. In FIG. 14, (1) illustrates in-phase axis output of the quadrature detector. Here, an n-th OFDM symbol consists of a guard part Gn (sample number: Ng) and a significant symbol part Sn (sample number: Ns), and signal Gn in the guard interval is a copy from a part Gn' of the significant symbol. In FIG. 14 (2), the signal shown in FIG. 14 (1) is delayed by the significant symbol duration. FIG. 14 (3) is a result of obtaining correlation between the two signals (1) and (2), by multiplying the signals (1) and (2), and then calculating a moving average in the width of the guard interval (Ng samples). As shown in FIG. 14 (3), since Gn and Gn' have the same signal waveform, the correlation output has peaks at boundaries of symbols.
Assuming that the peak value for the correlation between the in-phase axis data and the in-phase axis data delayed by the significant symbol duration is Sii, and that a peak value for correlation between the in-phase axis data and quadrature axis data delayed by the significant symbol duration is Siq, frequency error .delta. is obtained by the following equation: ##EQU1##